The Method of Measuring No-load Current of Three-phase Asynchronous Motor with Clamp Ammeter
Example 1 An ore crusher with a drive motor of 15kW. After the motor is overhauled, it runs normally without load, but it cannot be loaded. Once the load is added, the motor will trip due to overload. After inspection, the mechanical and power supply are all normal. The DC resistance of the motor coil is 2.4Ω, 3.2Ω and 2.4Ω respectively; the three-phase no-load current measured by the clamp ammeter is 9A, 5A and 8.8A respectively. It is certain that the motor coil has Fault. After removing the motor end cover, it was found that one of the wire ends of one of the phase windings had been loosened, and the solder had melted. The motor is double-wired, one of which is disconnected and the other is still connected, so the torque is reduced, and it can only rotate without load, but it cannot carry the load.
Example 2. There is a motor with a rated power of 13kW. The coil is rewound and the test machine is tested. When the motor runs without load, the speed is normal. After the load is on, the motor speed is very slow, or even does not rotate. The measured power supply voltage and the resistance of each phase are normal, and the three-phase no-load current is basically balanced when measured with a clamp meter, but the current values are all small, so it is concluded that the winding connection is wrong. Opening the end cover, it was found that the motor with the △ connection was wrongly connected to the Y connection, which made the normal running torque too small to carry the load, because the torque of the Y connection was one-third of that of the △ connection.
Example 3 A machine tool uses a 4kW motor. After the power is turned on, the motor does not rotate and only hums. Remove the motor wires, test that there is electricity on the power supply side, the three-phase voltage is also normal, the DC resistance of the winding is also balanced, the insulation is qualified, and the mechanical rotation is flexible. Finally, measure the no-load current with a clamp ammeter on the motor leads on the lower side of the switch. As a result, there is current in two phases and no current in one phase. Indicates a faulty wire in the conduit. Pull out the inner wire of the steel pipe, and find that a section of the wire has been basically broken, facing each other like two needle points, and there is white oxidized powder at the end of the wire. This is due to the excessive tension when passing through the pipe, the wire is thinned and elongated, and the long-term energized current heats up and oxidizes at the place that seems to be broken. At this time, the voltage can still be measured on the energized wire head, but the current cannot pass.
